Random interval timer

ABSTRACT

Apparatus according to the present invention produces an output at random time intervals following a Poisson distribution. A first pulse generator means, comprising first and second time delay circuits, produces a first periodic pulse having a period on the order of minutes. A second pulse generator means provides a second repetitive pulse having a predetermined average period which is much less than the period of the first pulse, with the exact time of occurrence of the second pulse being modulated by a random noise signal. A coincidence detector indicates time coincidence between the first and second pulses and causes an audible output to be produced. The duty factor of the second pulse predetermines the chance for coincidence.

Thomas H. Charters 6855 S.W. Raleighwood Way, Portland, Oreg. 97225 Mar.7, 1969 Apr. 6, 1971 Inventor Appl. No. Filed Patented RANDOM INTERVALTIMER 13 Claims, 5 Drawing Figs.

US. Cl 331/78, 307/232, 307/273, 307/283, 307/288, 331/47,331/lll,33I/lI3,33l/ll7,33I/l73 OTHER REFERENCES White, Journal ofScientific Instruments, Vol. 41, Jun. 1964, pp, 361- 364. (331-78)White, The Review of Scientific Instruments, Vol. 30, Sept. 1959, pp.s25 829. (331-78) Primary Examiner.lohn Kominski AssistantExaminerSiegfried H. Grimm Att0rneyBuckh0rn, Blore, Klarquist andSparkman ABSTRACT: Apparatus according to the present invention producesan output at random time intervals following a Poisson distribution. Afirst pulse generator means, comprising first and second time delaycircuits, produces a first periodic at. pulse having a period on theorder of minutes A eond pulse Flld of Search 331/47, 78, generator meansprovides a econd repetitive pulse having a 1 173; 328/63 predeterminedaverage period which is much less than the R f (fled period of the firstpulse, with the exact time of occurrence of e erences the second pulsebeing modulated by a random noise signal. A UNITED STATES PATENTScoincidence detector indicates time coincidence between the 2,671,8963/1954 DeRosa 343/ 17.1 first and second pulses and causes an audibleoutput to be 2/1969 Etter 33 I/78X produced. The duty factor of thesecond pulse predetermines 3,445,591 5/1969 Koehler et al. 33 I /78X thechance for coincidence.

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THOMAS H. CHARTERS INVENTOR mm ML KOF mwZmU mW OZ 2002 11 BUC/(HOR/V,BLORE, KLAROU/S T 8 SPAR/(MAN ATTORNEYS RANDOM INTERVAL TIMER BACKGROUNDOF THE INVENTION Statistical sampling is a method for gatheringinformation from a large body of data, and producing information whichis truly representative of the data. The accuracy of this kind ofsampling technique depends in each case upon the randomness whichcharacterizes the time intervals between samples. Heretofore, a persontaking random samples might record a sample, e.g. an activity at a giventime, and then consult a table of random time interval numbers forestablishing the intervening period before the next sample is taken.

Various semiautomatic timer devices are available which attempt toestablish random time intervals. However, rather than being trulyautomatic, some of these devices depend upon human intervention, suchas, for example, the setting of the device at the start of each timeinterval. Moreover, the number of samples taken over a given period oftime, that is, the average interval length, may not be statisticallycontrollable.

SUMMARY OF THE INVENTION According to the present invention, a firstmeans generates a first repetitive pulse having a period between pulseoccurrences on the order of minutes or greater. A second meansforgenerating a second repetitive pulse, having a predetemtined averageperiod which is much less than the period of the first pulse, generatessuch second pulse having a predetermined duration as compared with theaverage period between second pulses. The chance of time coincidencewith the first pulse'is thereby established. Means modulate the time ofoccurrence of one of the pulses with random noise, and time coincidencebetween the first and second pulses is detected for producing an audiotone or the like. As a consequence of the average periodicity of thesecond pulse and the predetermined dura' tion thereof as compared withthe average period between second pulses, a controlled Poissondistribution of outputs is achieved.

According to a preferred embodiment of the present invention, the meansfor producing the first pulse comprises first and second time delaycircuits wherein the first time delay circuit periodically produces anoutput with a relatively long interval between outputs. The second timedelay circuit is responsive to the output of the first and produces thesaid first pulse in a relatively short period of time after the outputof the first time delay circuit. The first pulse, as applied to acoincidence detector or the like, is then less subject to beinginfluenced by the occurrence of the aforementioned second pulse.

In a preferred embodiment of the present invention, the time ofoccurrence of the second pulse is modulated with a random noise signal.In accordance with a further embodiment of the present invention, theaforementioned second means for generating a second pulse is empoweredonly for a period of timewhen the aforementioned first time delaycircuit produces an output. The power consumption of the ap paratus.normally quite low, is then even lower.

It is accordingly an object of the present invention to provide animproved random interval timer which automatically provides outputindications at random time intervals.

It is a further object of the present invention to provide an improvedrandom interval timer characterized by a predetermined statisticaldistribution of interval times, averaging to a controlled number ofsamples.

It is another object of the present invention to provide an improvedrandom interval timer producing time interval outputs substantiallyfollowing a Poisson distribution.

It is a further object of the present invention to provide an improvedrandom interval timer which is portable in nature and which employs asmall power supply battery with a very small drain being made thereupon.

It is a further object of the present invention to provide an improvedrandom interval timer which is economical in construction, portable innature, and reliable in operation.

The subject matter which I regard as my invention is particularlypointed out and distinctly claimed in the concluding portion of thisspecification. The invention, however, both as to organization andmethod of operation, together with further advantages and objectsthereof, may best be understood by reference to the followingdescription taken in connection with the accompanying drawings whereinlike reference characters refer to like elements.

DRAWINGS FIG. I is a block diagram of apparatus according to the presentinvention;

FIG. 2 is a schematic diagram further illustrating the FIG. 1 apparatus;

FIG. 3 is a chart of waveforms illustrating operation of the apparatusof FIGS. 1 and 2;

FIG. 4 is a schematic diagram of a portion of the FIG. 2 circuitaccording to an alternative embodiment of the present invention; and

FIG. 5 is a graph illustrating a statistical distribution of intervaltimes provided by the timer according to the present invention.

DETAILED DESCRIPTION Referring to FIG. 1, illustrating a circuitaccording to the present invention in block diagram form, .a randomnoise generator 10 produces a noise signal indicated at A for timemodulating the output of rectangular wave generator 12. Rectangulargenerator 12 has a normal period in the absence of the noise input Awhereby it produces a train of rectangular waves having a given periodbetween occurrences thereof, if it were not affected by the A input.However, the A noise input is effective for either advancing orretarding the occurrence of each pulse output. As a result, an outputsignal illustrated at B is produced, comprising negative-going pulsesoccurring at substantially random times, but wherein the average periodbetween occurrences thereof is predetermined. This waveform Bconstitutes one input of coincidence detector I4. The wavefonn outputsillustrated next to the various blocks in FIG. I are duplicated in timedrelation in the wavefonn chart of FIG. 3.

The second input of coincidence detector 14 is provided from a pulsegenerator means comprising timing ramp circuit 16, sampling ramp gate18, and sampling ramp circuit 20. Timing ramp circuit I6 is a rampgenerator internally providing a waveform C. Circuit 16 produces a pulseoutput periodically with a relatively long time delay period betweenrecurrences, typically onthe order of minutes or longer. The output ofthe timing ramp circuit 16 is indicated at D and comprises apositive-going output pulse produced at the conclusion of each of theramps indicated in waveform C, for example as encircled in FIG. 1. Theoccurrence of waveform D operates sampling ramp gate 18 to produce agate output E which starts sampling ramp circuit 20. Sampling rampcircuit 20 is also a time delay circuit for producing output pulses at atime which is delayed from the occurrence of pulse D and gate E.Sampling ramp circuit 20 comprises a ramp generator for providing a rampwaveform F, and waveform G at the conclusion of such ramp. Whereas theduration of a ramp in waveform C is on the order of minutes or longer,typically being 3 to l2 minutes, the duration of ramp F is much shorter,being on the order of IO milliseconds in the specific example. Becauseof the disparity in the duration of these waveforms, the conclusion ofwavefonn C (e.g. as within the circled portion in FIG. 1) appears as arectangular wave in FIG. 3, when displayed on the same time scale withramp F.

When coincidence detector 14 detects the time coincidence of anegative-going pulse in waveform B and a pulse G, as indicated by dashedlines 22 in FIG. 3, the coincidence detector provides an output pulse Hfor operating oscillator gate 24 which in turn empowers oscillator 26for driving speaker 28 with a waveform J, the latter having a frequencywithin the audio range. Gate waveform l suitably has a duration ofapproximately 3 seconds, and only the part thereof circled in FIG. 1 isillustrated in FIG. 3. When an audio tone occurs. the person employingthe present device records a sample or the like. Of course, automaticsampling equipment may be energized in place of generation of an audiotone.

Considering the overall operation of the FlG. l circuit, timing rampcircuit 16, sampling ramp gate 18, and sampling ramp circuit 20 may beconsidered as a first means for providing a pulse output which isperiodic, wherein the period between repetitions of waveform G is quitelong, on the order of minutes or longer. Timing elements selected fortiming ramp circuit 16 and sampling ramp circuit 20 render this periodpredictable and selectable. On the other hand, the pulse output B fromrectangular generator 12 has a much shorter period of approximatelybetween I and 2 milliseconds, with this period varying randomlyaccording to noise input A. Therefore, not every occurrence of pulsewaveform G will coincide with a pulse waveform B to provide acoincidence.

The chance of coincidence occurring is determined by selecting theduration of the negative-going pulse outputs of rectangular wavegenerator 12. For example, if the duration of each negative-going pulseof waveform B were half of the period between pulse occurrences inwavefonn B, then there would be a 50 percent chance of a coincidenceoccurring. In a preferred circuit according to the present invention,the dura tion of each negative-going pulse B is between and I5 percentof the period between pulse occurrences, and preferably one-tenth theperiod. Therefore, there is about a percent chance that a given waveformoutput G will coincide with the waveform output B. Then, on the average,one coincidence will take place for every l0 occurrences of waveform G,but the time of coincidence cannot be predicted. The sampling principleemployed is analogous to flipping a coin with one head and nine tails.On the average, one out of ten tosses will produce a head. This type ofrandom process is described by a binomial probability law. However, thepresent circuit, employing a 10 percent duty factor rectangular wave l3and thereby establishing a probability of coincidence of 0.l at eachsampling time, causes the binomial probability law to reduceapproximately to the Poisson probability law.

The curve in FIG. 5 illustrates both a theoretical Poisson distributionas well as approximately the measured distribution of outputs from theFIG. 1 circuit. The graph plots the probability of one and only oneoutput occurring in a time, t. The average signal period is indicated bythe curve maximum. The intervals of outputs may actually range from 3minutes to as much as 8 hours, but the average interval length over manyintervals (eg more than intervals) is predictable and programmable. Inthe present circuit, the average interval length is predetermined bypresetting the duty factor of waveform B, or in other words, theduration of the negative-going pulses compared with the time betweenoccurrences thereof, and also by presetting the time between occurrencesof timing ramp circuit 16. This feature enables one to know in advancehow many audible outputs will occur at speaker 28 over an extendedperiod of time without knowing when the audible outputs will occur.Thus, the mean rate of occurrence and the range of time spreadtherebetween can be present so that a desired number, but not anexcessive number, of sampling times are predetermined. When the numberof sampling times can be predetermined, the person recording sampleswill not be interrupted from his regular between-sample activity by anexcessive amount.

The average periodicity of waveform B and the predetermined duty factorthereof contribute to providing a Poisson distribution, and controlthereof. The Poisson distribution is desirable or required for manytypes of statistical samplings. The present circuit is automatic andcontinues to produce the output having a Poisson distribution withoutresetting or other human intervention, thereby rendering the outputdistribution substantially foolproof.

One important feature of the FIG. 1 circuit relates to the employment ofa long duration ramp signal or timing period as indicated register C,which triggers a relatively short duration ramp or timing signal asindicated at F. The apparatus according to the present invention isquite portable, and in particular instance has provided throughdimensions 150, the emitterbase junction of transistor 156, diode 154,resistor 146, and transistor to ground, turning transistor 156 1% inches2% inches 5% inches, weighing 8 ounces. The device was battery operatedfrom a small transistor radio-type battery having a life of severalhundred hours under continuous operating conditions. As will beobserved, the slope of the timing ramp C is quite small when viewed onthe same time scale which illustrates the occurrence of the othersignals. (FIG. 3). It has been found that direct application of theoutput of a long duration timing ramp circuit to a coincidence detectorin a small apparatus frequently causes a coincidence to be indicatedbetween the timing ramp output and one of the negative-going pulses ofwaveform B, whether or not such coincidence should occur. The couplingin the small portable apparatus is relative ly close, especially whenthe same battery is employed for a power source leading to triggering ofthe end of a ramp in circuit 16 by one of the negative-going pulses ofwaveform B. According to the present circuit, however, the output oftiming ramp circuit 16 is applied via sampling ramp circuit 20 and viasampling ramp gate 18. A waveform F is produced having a much greaterslope, and the end of which has substantially no possibility of beingtriggered by the occurrence of one of the negative-going pulses ofwaveform B. Thus, a true statistical chance is present for thecoincidence of the output of sampling ramp circuit 20 and that ofrectangular wave generator 12. Even if the conclusion of a waveform Cramp is then triggered by one of the negative-going pulses of waveformB, there should still be a correct statistical chance of waveform Gcoinciding with one of the negative-going pulses of waveform B. In onecircuit, as hereinafter more fully described, operation of random noisegenerator 10 and rectangular wave generator 12 is actually started atthe conclusion of a waveform C ramp, thereby materially reducing batterydrain and prolonging the life of the battery to thousands of hours.

A specific example of the circuit according to the present invention isillustrated in FIG. 2 wherein like elements are referred to with likereference numerals. Random noise generator 10 comprises a transistor 30,the shot noise and the like from which is amplified greatly bytransistors 32 and 34 in a direct-coupled cascaded circuit. A DCfeedback path for this high gain amplifier is provided via resistors 36and 38 connected in series from the collector of transistor 34 to thebase of transistor 30. The center tap between resistors 36 and 38 isreturned to ground employing a relatively large capacitor 40. A resistor42 returns the base of transistor 30 to ground while load resistors 44,46, and 48 couple the respective transistor collectors to voltage supplyterminal 50. The output of transistor 34 is coupled through resistor 52and capacitor 54 in series to the base of transistor 56, the latterforming one component of rectangular wave generator 12.

The base of transistor 56 is also connected to the midpoint of a biasingvoltage divider comprising resistors 58 and 60 disposed between groundand positive voltage terminal 50, while a voltage divider comprisingresistors 62 and 64 in series returns the collector of transistor 56 toground. The collector of transistor 56 is also directly connected to thebase of transistor 66, the latter having its collector grounded and itsemitter coupled to the emitter of transistor 56 via capacitor 68.Resistor 70 connects the emitter of transistor 56 to voltage supplyterminal 50 and a variable resistor 72 similarly connects the emitter oftransistor 66 to voltage supply terminal 50.

The rectangular wave generator 12 as thus far described comprises afree-running multivibrator wherein the transistors 56 and 66 arealternately conducting. Resistor 70 is typically much larger thanresistor 72, with resistor 70 suitably having a value of approximately Kwhile resistor 72 may have an average value of 20 K. Assuming transistor56 has just become conducting, a positive-going signal will be appliedto the base of transistor 66 via the direct connection, and anegative-going signal will be applied to the emitter of transistor 66via capacitor 68. Transistor 66 will therefore shut off. Current throughresistor 72 will now charge capacitor 68 whereby the emitter oftransistor 66 eventually becomes positive enough so that transistor 66begins to conduct. The resulting negative-going voltage excursion on theemitter of transistor 66 is coupled to the emitter of transistor 56through capacitor 68 tending to turn transistor 56 off. As transistor 56conducts less current, the collector voltage thereof drops wherebytransistor 66 conducts more strongly, and so on. Now, resistor 70charges capacitor 68 until the emitter of transistor 56 becomes positive enough to turn on transistor 56. Since resistor 70 is larger thanresistor 72, transistor 66 conducts the majority of the time. Resistor72 is adjusted so that transistor 56 conducts about percent of the time,for an appropriate value to provide the aforementioned Poissondistribution. Of course, other values may also be selected for adjustingthe duration of pulses in waveform B.

The circuit comprising transistor 56 and 66 is free-running, andnormally provides a positive-going, approximately 10 percent duty factorpulse, which is substantially periodic, at the base of transistor 74.However, noise signals coupled via resistor 52 and capacitor 54 retardor advance the occurrence of such outputs as hereinbefore mentioned.Thus transistor 56 may become triggered at its base at a time before itnormally would, or the noise signal may be of an opposite polarity,tending to delay triggering. The pulse outputs are inverted intransistor 74 and applied at the emitter of transistor 76 in coincidencedetector 14. Thus, negative-going pulses are applied at the emitter oftransistor 76, such emitter being biased by means of a voltage dividercomprising resistor 78 and 80 disposed between voltage supply terminal50 and ground.

Timing ramp circuit 16 employs a four-layer semiconductor device knownas Programmable Unijunction Transistor," manufactured by the GeneralElectric Company. This element is indicated by reference numeral 84, andincludes an anode" terminal A, a cathode" terminal K, and a "gate"terminal G. In the specific circuit, the Programmable UnijunctionTransistor" was a General Electric type DI3T2. The transistor 84operates such that a fairly high resistance exists between terminals Aand K thereof until such time as the voltage applied at terminal Aexceeds the voltage applied at terminal G. Further referring to thediagram, a time constant circuit comprising resistor 86 and capacitor 90is disposed in that order between a positive voltage point and ground.The junction between resistor 86 and capacitor 90 is connected toTerminal A of transistor 84, while terminal K of transistor 84 isreturned to ground through resistor 92. Terminal G of transistor 84 isconnected to the midpoint of a voltage divider comprising resistor 95,and one of the selected resistors 96 through 103 disposed in that orderbetween a positive voltage point and ground. A jumper 104, suitablyaccessible in the instrument, is employed for selecting one of theresistors 96 through 103. The various resistors 96 through 103 aregraded resistances, and provide a selectably lower voltage at terminal Gof transistor 84 as the jumper 104 is moved to the right. Let us assumecapacitor 90 starts to charge through resistor 86. One of the rampwaveforms indicated at C in FIG. 1 will be produced as the capacitorcharges, and when it reaches the voltage of terminal G as selected bythe voltage divider 95, 96- 103, the transistor device 84 exhibits a lowresistance between terminals A and K whereby capacitor 90 is immediatelydischarged. The circuit comprising resistor 86 and capacitor 90 isarranged to have a selectable long-time constant on the order of severalminutes or longer as hereinbefore mentioned, and when the voltage ofterminal G is reached by capacitor 90, then a current increase will takeplace in resistor 92 resulting in a positive-going pulse as indicated atD in FIGS. 1 and 3 which is applied via resistor 106 to the base oftransistor 108 in sampling ramp gate 18.' In order to achieve an evenlonger time constant, an additional capacitor (not shown) may beinserted at tenninals 105 across capacitor 90.

The sampling ramp gate is a monostable circuit comprising transistors108 and 110 which are both either off or on at the same time. Thesetransistors are normally in a nonconducting condition. The base oftransistor 108, in addition to being connected to resistor 106, is alsoconnected to a voltage divider comprising resistors 112 and 114 seriallydisposed between the base of transistor 108 and ground. The midpoint ofthis voltage divider is connected to the collector of transistor 1 10.The emitter of transistor 110 is coupled to a positive source throughresistor 116 and the capacitor 118 is disposed between the emitter andground. The collector of transistor 108 and the base of transistor 110are returned to a positive source employing resistors 120 and 122,respectively. A feedback coupling circuit comprises a capacitor 124 inseries with a resistor 126, the series connection being shunted by acapacitor 128. Such parallel connection is disposed between thecollector of transistor 108 and the base of transistor 110.

When the positive-going pulse is applied at the base of transistor 108at the conclusion of a ramp in waveform C, transistor 108 is turned on,and a negative-going excursion at its collector terminal is coupled tothe base of transistor 110 by way of elements 124, 126, and 128.Transistor 110 is also thereby turned on. When capacitors 124 and 128charge to a given value, transistor 110 will turn off, and its collectorvoltage will drop, and this drop, coupled to the base of transistor 108through resistor 112, turns transistor 108 off. During the time thattransistors 108 and 110 are on, a positive pulse E (see FIGS. 1 and 3)is produced. The period thereof is suitably about I second.

The pulse E is applied to a time constant circuit comprising resistor130 in series with capacitor 132 in sampling ramp circuit 20. Theelements 130 and 132 are disposed between the collector of transistor110 and ground, with the junction therebetween being directly coupled tothe A terminal of Programmable Unijunction Transistor" 134 which is alsoprovided with terminals G and K, these terminal designations having thesame meanings as hereinbefore described in connection with transistor84. Transistor 134 in a specific circuit was a General Electric typeD13Tl. The positive pulse from the collector of transistor 110 is alsoapplied to a voltage divider comprising resistors 135 and 136 interposedin that order between the collector of transistor 110 and ground. Thejunction between resistors 135 and 136 is direct coupled to terminal Gof transistor 134, while a capacitor 138, shunted to ground, stabilizesthe voltage at this point. Terminal K at transistor 134 is connected tothe base of transistor 140, in coincidence detector 14, while theparallel combination of resistor 142 and capacitor 144 is disposedbetween terminal K and ground. When the output pulse E is applied to thetime constant circuit comprising resistor 130 and capacitor 132, thecapacitor 132 charges up as indicated by waveform F in FIGS. 1 and 3.When the charge reaches a predetermined value set at terminal G by thevoltage at the midpoint of the voltage divider 135- 136, a lowresistance path is provided between terminals A and K of transistor 134,and capacitor 132 is discharged. Also at this time, a positive pulse isprovided at the base of transistor 140, as an input to the coincidencedetector 14.

This positive pulse is waveform G in FIGS. 1 and 3. The time constant ofthe circuit comprising resistor 130 and capacitor 132 is much shorterthan that of circuit 86-90, whereby waveform G is produced a fewmilliseconds, e.g. 10 milliseconds, after the start of waveform E. Incoincidence detector 14, the emitter of transistor is grounded, whilethe collector is connected to a positive voltage by way of resistors146, 148 and 150 in series. Resistor 152 in series with diode 154 isconnected across resistor 148, with the cathode of diode 154 beingconnected to the midpoint between resistors 146 and 148. The emitter oftransistor 156 is connected to the junction between resistors 148 and150, and the base of the same transistor is connected to the junctionbetween resistor 152 and the anode of diode 154. A capacitor 158 isdisposed between the emitter of transistor 156 and ground. The collectorof transistor 158 is returned to ground through resistor 160 and diodes162 in series, while the junction between resistor 160 and the firstdiode 162 is coupled to the base of transistor 76 by diode 164. Diodes162 and 164 have their anode terminals oriented toward the collector oftransistor 156. A resistor 166 is disposed between the anode of diode164 and ground, while another resistor 168 is disposed between thecathode of diode 164 and ground.

As hereinbefore mentioned, a negative-going output pulse fromrectangular wave generator 12 is applied at the emitter of transistor76. In order to register a coincidence, a positivegoing output fromsampling ramp circuit would simultaneously have to occur at the base oftransistor 140. If this should happen, a circuit would be providedthrough resistor 150, the emitter-base junction of transistor 156, diode154, resistor 146, and transistor 140 to ground, turning transistor 156on. The resultant positive-going excursion at its collector is coupledthrough resistor 160 and diode 164 to the base of transistor 76. Withboth a positive-going input applied to the base of transistor 76 and anegative-going input applied to the emitter of transistor 76, transistor76 will turn on, coupling a negative-going signal, indicated at H inFIGS. 1 and 3, to the base of transistor 176 in oscillator gate 24 viaresistor 170. If both a positive-going signal from sampling ramp circuit20 and a negative-going signal from rectangular wave generator 12 arenot simultaneously applied to coincidence detector 14,

' transistor 76 will not turn on. Diodes 162 and 164 provide means forapplying a stabilized voltage value to the base of transistor 76 when apositive output from sampling ramp circuit 20 is present Oscillator gatecircuit 24 comprises a monostable circuit wherein transistors 174 and176 are both either on at the same time or off at the same time. Bothtransistors are normally off. The emitter of transistor 174 is grounded,and its collector is connected to a positive voltage point via resistors178 and 172 in series. The midpoint between resistors 178 and 172 isalso connected to the base of transistor 176. The emitter of transistor176 is connected to a positive voltage while its collector isdirect-coupled to the base of transistor 180 as well as being returnedto ground by resistor 182. A resistor 184 returns the base of transistor174 to ground. A feedback circuit 3 disposed seconds. the base oftransistor 174 and the collector of 176, this circuit comprising theseries combination of a capacitor 186 and resistor 188 wherein theseries connection is shunted by a capacitor 190. Also, a capacitor 192is connected across capacitor 186.

When a coincidence is indicated by coincidence detector 14, thenegative-going pulse applied at the base of transistor 176 turnstransistor 176 on, and the resulting positive-going excursion at thecollector of transistor 176 is coupled through the feedback circuit tothe base of transistor 174, turning transistor 174 on. Thepositive-going signal at the collector of transistor 176 is also coupledthrough emitter-follower connected transistor 180 to oscillator 26. Thecollector of transistor 180 is connected to a positive voltage-employingresistor 194, and a capacitor 196 is disposed between the collector andground. The output at the emitter of transistor 180 is indicated at I inFIGS. 1 and 3, and suitably lasts for approximately 3 178 seconds.During this time, capacitors 186, 190, and 192 charge, and eventuallythe voltage at the base of transistor 174 lowers to a point at whichtransistor 174 ceases to conduct. The resultant positive-going excursionat the collector of transistor 174 is coupled by means of resistor 178to the base of transistor 176, cutting the latter off, and concludingthe output of oscillator gate 24.

Oscillator 26 comprises a Hartley oscillator circuit wherein atransistor 198 has its collector connected to the emitter of transistor180 and its emitter connected to the midpoint of the primary winding ona transformer 200. One end of the primary winding on transformer 200 isgrounded, while the nongrounded end is coupled through the seriesconnection of resistors 202 and 204 to the collector of transistor 198.The midpoint between the resistors 202 and 204 is connected to the baseof transistor 198. Also, a diode 206 is disposed between the emitter oftransistor 198 and the junction between resistors 202 and 204. The diode206, the anode of which is connected to the emitter of transistor 198,is employed to avoid reverse breakdown of the base-emitter junction oftransistor I98.

A capacitor 208 is connected across the primary winding of transformer200, and the circuit constant are such that oscillation takes place atabout 500 cycles per second to produce an 4 audio note at speaker 28 forthe duration of the output, I, from oscillator gate 24. Thecollector-emitter current of transistor 198 flows to ground in the lowerportion of the primary winding of transformer 200, while the upperportion of the primary winding provides feedback to the base oftransistor 198 whereby oscillation is sustained.

The secondary of transformer 200 is coupled to speaker 28 via theparallel combination of resistor 210 and variable resistor 212 as wellas through the normally closed contacts of headphone jack 214. Thus,oscillations are normally provided to speaker 28, and variable resistor212 can be used as a volume control. Alternatively, a headphone plug maybe inserted in jack 214 for operating a headphone and disconnectingspeaker 28.

A power supply for the apparatus comprises a battery 216, suitably asmall eight volt transistor radio battery, the negative terminal ofwhich is grounded. The positive terminal of the battery is connected viamain switch 218 and decoupling resistor 220 to the supply terminal 50for empowering the random noise generator 10 and the rectangular wavegenerator 12. A decoupling capacitor 222 is connected between ground andthe junction between switch 218 and resistor 220, and another decouplingcapacitor, 224, is connected between ground and terminal 50. Thepositive eight volt supply across capacitor 222 is provided for circuitblocks 14, 16, 18, 20, 24, and 26, while the positive 8 volt supplyterminal 50 is decoupled from the rest of the circuit. In this manner,noise and the operation of rectangular wave generator 12 are less liketo trigger the end of a sampling ramp in sampling ramp circuit 20. Ingeneral, rather large values of load resistors are employed in thevarious individual circuits in order to reduce battery drain. A mercurybattery is preferred because of the longer storage life when theinstrument is not used.

A circuit further reducing battery drain is illustrated in FIG. 4wherein primed reference numerals are employed to refer to componentssimilarly numbered in FIG. 2. The FIG. 4 circuit is substantially thatof sampling ramp gate 18 and operates as hereinbefore described. Thecircuit may be substituted for the sampling ramp gate in FIG. 2,according to an alternative embodiment of the present invention. Thepositive 8 volt supply, indicated in FIG. 4, is obtained from thejunction of switch 218 and capacitor 222, in common with the 8 voltsupply to most of the rest of the FIG. 2 circuitry. However, the supplyfor random noise generator 10 and rectangular wave generator 12 is nowtaken at terminal 230 through an additional transistor 226 connected toterminal 50', and via resistor 220' from the 8 volt main supply.Transistor 226 is energized via resistor 228 from the collector oftransistor 108', only when the sampling ramp gate provides the waveformE. As a result, the random noise generator and rectangular wavegenerator will not be energized until waveform D is produced at the endof the timing ramp, consequently reducing battery drain and extendingbattery life to thousands of hours. When waveform output B is theninitiated from rectangular wave generator, the period thereof betweennegative-going pulses will be random as determined by the noisegenerator, and the subsequent coincidence with waveform G may or may notoccur, as hereinbefore described, on a statistical basis. The generaloperation of the circuitry of FIGS. 2 and 4 is otherwise substantiallythe same as described in connection with FIG. 1.

While I have shown and described preferred embodiments of my invention,it will be apparent to those skilled in the art that many changes andmodifications may be made without departing from mu invention in itsbroader aspects.

Iclaim:

1. Apparatus for producing an output at random, unpredictable intervalswherein the average number of such outputs over an extended period oftime can be predetermined, said apparatus comprising:

first means for generating a first repetitive pulse having a periodbetween pulse occurrences on the order of minutes or greater;

second means for generating a second repetitive pulse having apredetennined average period which is much less than the period of saidfirst pulse, said second pulse at each repetition thereof having apredetermined duration as compared with the average period between saidsecond pulses for establishing a controlled chance of time coincidencewith a said first pulse;

means for modulating the final time of occurrence of one of said pulsesby means of random noise; and

means for detecting time coincidence between said first and secondpulses for providing said output.

2. The apparatus according to claim 1 wherein the final time ofoccurrence of said second pulse is determined by said random noise, withthe average time of occurrence thereof remaining substantially constant.

3. The apparatus according to claim 1 wherein the duration of eachsecond pulse is between 5 and percent of the period between occurrencesthereof to establish a chance of time coincidence of corresponding valueand to establish substantially a Poisson distribution of coincidences.

4. The apparatus according to claim 1 wherein said first means forgenerating said first pulse comprises a first time delay circuit forproviding a substantially periodic output having a delay on the order ofminutes or greater between occurrences of such substantially periodicoutput, and a second time delay circuit operated by said first timedelay circuit for producing a much shorter delay and providing saidfirst pulse at the end of said shorter delay.

5. Apparatus for producing an output at random, unpredictable intervalscomprising:

a first means for providing a first repetitive pulse, said first meanscomprising a first time delay circuit generating a repetitive output anda second time delay circuit operated by said first time delay circuitand providing an output corresponding to said first pulse at a timeafter the output of said first time delay circuit shorter than the timebetween repetitions of the output of the first time delay circuit;second means for providing a second repetitive pulse having a muchfaster repetition rate than said first pulse; and

means for registering a coincidence between the occurrence of said firstpulse and the occurrence of said second pulse to provide an output forsaid apparatus.

6. The apparatus according to claim 5 wherein said first time delaycircuit provides a periodic output having a period on the order ofminutes, and said second time delay circuit provides'a delay for theproduction of said first pulse which delay is on the order ofmilliseconds.

7. The apparatus according to claim 5 wherein said second means furtherincludes a random noise generator for modulating the output time ofrepetitions of said second pulse.

8. The apparatus according to claim 5 further including an audiooscillator for providing an audible tone in response to a coincidenceregistered by said means for detecting such coincidence.

9. The apparatus according to claim 5 wherein said first time delaymeans is connected to empower said second means at the time of an outputof the first time delay means.

10. The apparatus according to claim 5 including a common power supplyfor said first and second means.

11. The apparatus according to claim 5 wherein said first and secondtime delay circuits each comprise ramp waveform generators includingmeans for detecting when a ramp wavefomr generated thereby reaches apredetermined value for ascertaining a predetermined time, the rampproduced by the second time delay circuit having a greater slope thanthe ramp tirloduced by the first time delay circuit.

1 e apparatus according to claim 7 wherern said second means furthercomprises a rectangular wave generator having a normal predeterminedperiod and receiving the output of said noise generator for modulatingsuch period.

13. The apparatus according to claim 12 wherein said rectangular wavegenerator is adapted to produce said second pulse with a predeterminedduration compared with the period between repetitions thereof forestablishing a duty factor for predicting the number of coincidencesdetected by said means for detecting such coincidences, over an extendedperiod of time.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,573,652 Dated A n 6, 1971 Inventor(s) THOMAS H. CHARTERS It is certifiedthat error appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

Column 3, line 59, "present" should read --preset-- Column 4, line I,"register" should read --at--; line 3 after "and in" insert --a--; lines4 through 6, "has provid through dimensions 150, the emitter-basejunction of transi 156, diode 154, resistor 146, and transistor toground, tur transistor 156" should read --had case dimensions of--Column 7, line 1, "158" shouldread --l56--; line 30, i a period, after"present" line 42, "3% disposed sec should read --is disposed between-;line 59, "3 178" shoul read --three and onehalf-- Column 8, line 8,"constant" should be --constants--; line 37, "like" should be-'---likely---; line 73, "mu" should --my--.

Signed and sealed this 9th day of November 1971.

(SEAL) Attest:

EDWARD M.FLEICHER,JR. ROBERT GOTTSCHALK Attesting Officer ActingCommissioner of PM

1. Apparatus for producing an output at random, unpredictable intervalswherein the average number of such outputs over an extended period oftime can be predetermined, said apparatus comprising: first means forgenerating a first repetitive pulse having a period between pulseoccurrences on the order of minutes or greater; second means forgenerating a second repetitive pulse having a predetermined averageperiod which is much less than the period of said first pulse, saidsecond pulse at each repetition thereof having a predetermined durationas compared with the average period between said second pulses forestablishing a controlled chance of time coincidence with a said firstpulse; means for modulating the final time of occurrence of one of saidpulses by means of random noise; and means for detecting timecoincidence between said first and second pulses for providing saidoutput.
 2. The apparatus according to claim 1 wherein the final time ofoccurrence of said second pulse is determined by said random noise, withthe average time of occurrence thereof remaining substantially constant.3. The apparatus according to claim 1 wherein the duration of eachsecond pulse is between 5 and 15 percent of the period betweenoccurrences thereof to establish a chance of time coincidence ofcorresponding value and to establish substantially a Poissondistribution of coincidences.
 4. The apparatus according to claim 1wherein said first means for generating said first pulse comprises afirst time delay circuit for providing a substantially periodic outputhaving a delay on the order of minutes or greater between occurrences ofsuch substantially periodic output, and a second time delay circuitoperated by said first time delay circuit for producing a much shorterdelay and providing said first pulse at the end of said shorter delay.5. Apparatus for producing an output at random, unpredictable intervalscomprising: a first means for providing a first repetitive pulse, saidfirst means comprising a first time delay circuit generating arepetitive output and a second time delay circuit operated by said firsttime delay circuit and providing an output corresponding to said firstpulse at a time after the output of said first time delay circuitshorter than the time between repetitions of the output of the firsttime delay circuit; second means for providing a second repetitive pulsehaving a much faster repetition rate than said first pulse; and meansfor registering a coincidence between the occurrence of said first pulseand the occurrence of said second pulse to provide an output for saidapparatus.
 6. The apparatus according to claim 5 wherein said first timedelay circuit provides a periodic output having a period on the order ofminutes, and said second time delay circuit provides a delay for theproduction of said first pulse which delay is on the order ofmilliseconds.
 7. The apparatus according to claim 5 wherein said secondmeans further includes a random noise generator for modulating theoutput time of repetitions of said second pulse.
 8. The apparatusaccording to claim 5 further including an audio oscillator for providingan audible tone in response to a coincidence registered by said meansfor detecting such coincidence.
 9. The apparatus according to claim 5wherein said first time delay means is connected to empower said secondmeans at the time of an output of the first time delay means.
 10. Theapparatus according to claim 5 including a common power supply for saidfirst and second means.
 11. The apparatus according to claim 5 whereinsaid first and second time delay circuits each comprise ramp waveformgenerators including means for detecting when a ramp waveform generatedthereby reaches a predetermined value for ascertaining a predeterminedtime, the ramp produced by The second time delay circuit having agreater slope than the ramp produced by the first time delay circuit.12. The apparatus according to claim 7 wherein said second means furthercomprises a rectangular wave generator having a normal predeterminedperiod and receiving the output of said noise generator for modulatingsuch period.
 13. The apparatus according to claim 12 wherein saidrectangular wave generator is adapted to produce said second pulse witha predetermined duration compared with the period between repetitionsthereof for establishing a duty factor for predicting the number ofcoincidences detected by said means for detecting such coincidences,over an extended period of time.